Final answer:
The motor end-plate is a specialized part of the muscle fiber sarcolemma that contains receptors for acetylcholine at the neuromuscular junction, which is essential for muscle contraction.
Step-by-step explanation:
The specialized underlying portion of the muscle cell membrane at the neuromuscular junction, which contains receptors for the neurotransmitter acetylcholine, is known as the motor end-plate. The motor end-plate is a critical structure for muscle contraction, as it receives signals from the nerve that prompt the muscle to contract. The sarcolemma is the plasma membrane of the muscle fiber. It encapsulates the components inside the muscle cell, including the sarcoplasm (the cytoplasm of muscle cells) and the sarcoplasmic reticulum (SR), which is responsible for storing, releasing, and retrieving calcium ions necessary for muscle contraction.
The motor end-plate lies on the sarcolemma at the neuromuscular junction (NMJ), where the axon terminal of a neuron releases acetylcholine (ACh). When ACh is released, it diffuses across the synaptic cleft and binds to the receptors on the motor end-plate, initiating the sequence of events that lead to muscle contraction.